1. Technical Field
This invention relates to the technology of influencing the catalytic activity of metal oxides useful in gas sensors and/or catalytic converters, and more particularly to the technology for increasing or decreasing catalytic activity of TiO.sub.2 for oxidation of oxidizable molecules such as carbon monoxide and hydrocarbons.
2. Discussion of the Prior Art
Metal oxides, such as TiO.sub.2, have been used as materials for gas sensors and catalysts when promoting oxidation of gas molecules such as CO, H.sub.2, and HC. Such metal oxides are not selective as catalysts or as chemical sensor materials, that is, they catalyze or sense much more than one chemical at the same time. It would be desirable to achieve some degree of selectivity by such metal oxides without contaminating the material and/or reducing its effectiveness.
U.S. patents disclosing use of TiO.sub.2 in automotive catalyst systems have restricted its function to a barrier or stabilizer coating over other elements, which may be conductive, the latter acting as the catalyst.
U.S. Pat. No. 4,650,782 suggests that titania may be used as a thick protective outer layer over a catalytically effective amount of a noble metal (i.e., platinum) which in turn is supported by alumina. All layers, including the titania and alumina, are coatings achieved by dipping in a slurry. The resulting TiO.sub.2 coating is 0.003-0.01 inches thick, preventing its use as a modifier of catalyst selectivity. The titania is not used as an active catalyst, but rather as a barrier to lead poisoning in the presence of SO.sub.2. Such reference does not teach how to activate titania as an oxidizing catalyst, particularly for use as an electrode in gas sensors.
U.S. Pat. No. 4,478,797 is another automotive exhaust gas converter disclosure which deposits a mixture of catalytically active amounts of platinum and titania over a metallic substrate, but is separated by an intervening insulating washcoat layer of alumina. Data in such patent shows that there is no difference in the efficiency between catalysts with or without titania for conversion of CH.sub.4 and C.sub.2 H.sub.4 (see FIG. 3). Accordingly, this reference does not teach how to activate titania as an oxidizing catalyst nor make TiO.sub.2 more selective to certain gases.